Hydraulic control system for automatic transmission

ABSTRACT

A single accumulator is operatively connected to a fluid operated clutch so as to retard a rise in fluid pressure supplied to the fluid operated clutch which is to be engaged during a downshift from the highest speed ratio to any one of the lower speed ratios and when the transmission shifts from a neutral to a forward drive.

BACKGROUND OF THE INVENTION

The present invention relates to a hydraulic control system for anautomatic transmission.

It is a well known technique to provide an accumulator in a hydrauliccontrol system for an automatic transmission so as to alleviate a shockwhich takes place inherently during shift by gradually increasing afluid pressure supplied to fluid operated friction units. In the case ofa conventional automatic transmission, for example, a four speedautomatic transmission, in order to alleviate a shock taking placeduring a downshift from the four speed ratio to the third speed ratio,the second speed ratio or the first speed ratio (i.e., 4--3 downshift,4--2 downshift, 4--1 downshift) and a shock taking place when a manualvalve is moved from a "N" position thereof to a "D" position thereof, aplurality of accumulators are required, each being adapted to operateduring one of these shifts. However, providing the plurality ofaccumulators creates a problem that a hydraulic circuit becomescomplicated and a large space is required for the installation of theaccumulators.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a hydraulic controlsystem for an automatic transmission which has a single accumulator thatis operable to alleviate a shock taking place during a downshift fromthe highest speed ratio to any one of the lower speed ratios and a shocktaking place when the transmission shifts from a neutral thereof to aforward drive.

According to the present invention, there is provided an automatictransmission comprising:

a plurality of fluid operated friction units which are selectively madeoperative and inoperative to produce a plurality of forward speed ratiosincluding the highest speed ratio, the plurality of fluid operatedfriction units including a fluid operated clutch which is to be engagedduring a downshift from the highest speed ratio to any one of the lowerspeed ratios and when the transmission shifts from a neutral to aforward drive

means, including a fluid conduit communicating with the fluid operatedclutch, for supplying fluid pressure to the fluid operated clutchthrough the fluid conduit;

a pressure accumulator communicating with the fluid conduit, thepressure accumulator including means for retarding a rise in fluidpressure supplied to the fluid operated clutch through the fluidconduit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a power transmission mechanism of a powertransmission mechanism of a four speed automatic transmission asillustrated in connection with an engine which has a throttle mounted inan intake passage and an accelerator pedal;

FIGS. 2(a), 2(b), and 2(c), when combined, illustrate a circuit diagramof a first embodiment of a hydraulic control system for the automatictransmission according to the present invention;

FIG. 3 is a graph showing the variation in a throttle pressure versusthe degree of throttle opening and that in a throttle modulator pressureversus the degree of throttle opening;

FIG. 4 is a graph showing the variation in a pressure modifier pressureversus the degree of throttle opening;

FIG. 5 is a graph showing the variation in a cut back pressure versusrotational speed of a transmission output shaft for different degrees ofthrottle opening;

FIG. 6 is a graph showing the variation in a governor pressure versusrotational speed of the transmission output shaft;

FIG. 7 is a graph showing the variation in a line pressure versus thedegree of throttle opening for different rotational speeds of thetransmission output shaft;

FIG. 8 is a graph showing the variation of the line pressure versus therotational speed of the transmission output shaft for different degreesof throttle opening; and

FIG. 9 is shift pattern diagram of the automatic transmission.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is illustrated a power transmission mechanismof a four forward speed and one reverse speed automatic transmissionhaving an overdrive. This power transmission mechanism comprises aninput shaft I operatively connected via a torque converter T/C to anengine output shaft E of an engine which has a throttle which opens indegrees, an output shaft O operatively connected to road wheels, onlyone being shown, via a final drive, not shown. A first planetary gearset G1 and a second planetary gear set G2 are connected between theinput and output shafts I and O. A plurality of fluid operated frictionunits are provided which are made operative and inoperative forproducing a plurality of speed ratios between the input shaft I andoutput shaft O. The fluid operated frictional units include a firstclutch C1, a second clutch C2, a third clutch C3, a first brake B1, asecond brake B2, and a one-way brake OWC. The first planetary gear setG1 comprises a sun gear S1, an internal gear R1, a carrier PC1 carryingpinion gears P1 meshing simultaneously both the gears S1 and R1. Theplanetary gear set G2 comprises a sun gear S2, an internal gear R2 and acarrier PC2 carrying pinion gears P2 meshing simultaneouly both gears S2and R2. The carrier PC1 is connectable via the clutch C2 with the inputshaft I, and the sun gear S1 is connectable via the clutch C1 with theinput shaft I. The carrier PC1 is connectable via the clutch C3 with theinternal gear R2. The sun gear S2 is constantly connected with the inputshaft I. The internal gear R1 and carrier PC2 are constantly connectedwith the output shaft O. The brake B1 is arranged to anchor the carrierPC1. The brake B2 is arranged to anchor the sun gear S1. The one-waybrake OWC is so constructed that it allows forward rotation (i.e., thesame rotation as that of the engine output shaft E), but preventsreverse rotation (i.e., the rotation opposite to the forward rotation).Thus, it acts as a brake only during reverse rotation.

In the above mentioned power transmission mechanism, the rotationalstate of each of the rotary elements (S1, S2, R1, R2, PC1, and PC2) ofthe planetary gear sets G1 and G2 can be varied by actuating selectedone or combination of the cluthes C1, C2 and C3, brake B1, (one-waybrake OWC) and brake B2, thus varying the revolution speed of the outputshaft O relative to that of the input shaft I. The four forward speedratios and one reverse speed ratio are produced if the clutches C1, C2and C3 and brakes B1 and B2 are engaged in the manner as shown in thefollowing Table.

                  TABLE                                                           ______________________________________                                                             B1                  α.sup.1 = 0.45                 C1       C2    C3    (DWC)  B2  Gear Ratio                                                                             α.sup.2 = 0.45                 ______________________________________                                        1st speed          o   o                                                                                         ##STR1##                                                                              3.22                               2nd speed          o          o                                                                                  ##STR2##                                                                              1.38                               3rd          o     o              1        1                                  speed                                                                         4th speed    o                o                                                                                  ##STR3##                                                                              0.69                               Reverse                                                                              o               o                                                                                         ##STR4##                                                                              -2.22                              ______________________________________                                    

In the above Table, a sign "o" denotes actuation state of the clutch orbrake, α1 and α2 respectively denote ratios of number of teeth of theinternal gears R1 and R2 to number of teeth of the corresponding sungears S1 and S2. A gear ratio is a ratio of the revolution number of theinput shaft I to that of the output shaft O. What is denoted by thelabel (OWC) below the brake B1 means that the first speed ratio isproduced owing to the action of the one-way brake OWC even if the brakeB1 is not applied. However, in this first speed ratio, it is notpossible for the output shaft O to drive the engine (that is, no enginebraking is effected).

Referring to FIGS. 2(a), 2(b), and 2(c), a hydraulic control system forthe above power transmission mechanism is described.

This hydraulic control system comprises a regulator valve 2, a manualvalve 4, a throttle valve 6, a throttle fail safe valve 8, a throttlemodulator valve 10, a pressure modifier valve 12, a cut back valve 14, aline pressure booster valve 16, a governor valve 18, a 1-2 shift valve20, a 2-3 shift valve 22, a 3-4 shift valve 24, a 2-4 timing valve 26, a2-3 timing valve 28, a 3-4 timing valve 30, a 3-2 timing valve 32, afirst manual range pressure reducing valve 34, a torque converterpressure reducing valve 36, a 1-2 accumulator 38, a 4-3 accumulator 40,and an overdrive inhibitor solenoid 42. These valves are interconnectedas shown in FIGS. 2(a), 2(b), and 2(c), and connected with an oil pumpO/P, the torque converter T/C, clutches C1, C2 and brakes B1 and B2 asshown. The brake B2 has a servo apply chamber S/A, i.e., an oil pressurechamber designed to apply the brake when pressurized, and a servorelease chamber S/R, i.e., an oil pressure chamber designed to releasethe brake when pressurized. Since the servo release chamber S/R has alarger pressure acting area than a pressure acting area of the servoapply chamber S/A, the brake B2 is released when the pressure issupplied to the servo release chamaber S/R irrespective of the supply ofoil pressure to the servo apply chamber S/A. The overdrive inhibitorsolenoid 42 is electrically connected with an overdrive inhibitor switchSW.

The construction and operation of each of the valves are described.

Referring to FIG. 2(b), the regulator valve 2 comprises a valve bore 102formed with nine ports 102a to 102i and a spool 202 having formedthereon four lands 202a to 202d slidably disposed in the valve bore 102.It also comprises a stationary sleeve 252, formed with two ports 252aand 252b, fixed relative to the valve bore 102, a spool 203 havingformed thereon lands 203a and 203b slidably disposed in the stationarysleeve 252, and a spring 302 arranged between the land 202d of the spool202 and a spring seat 254 mounted on the upper end of the sleeve 252 asviewed in FIG. 2(b). The lands 202b, 202c and 202d are the same indiameter, and the land 202a is smaller in diameter than these lands. Theland 203a of the spool 203 is larger in diameter than the land 203bthereof. The ports 102a, 102c and 102g are drain ports. The ports 102 band 102e are connected with an oil conduit 402 (the line pressurecircuit). An orifice 606 is provided in the inlet to the port 102b. Theport 102d is connected via an oil conduit 404 with a variable volumechamber C/C of the oil pump O/P. The oil pump O/P is a vane pump of thevariable volume discharge type and is designed to decrease the dischargerate of the oil flow in response to an oil pressure supplied to the oilchamber C/C. The port 102f is connected via an oil conduit 406 with aport 136b of a torque converter pressure reducing valve 36 shown in FIG.2(a). An orifice 608 is provided in the oil conduit 406. The port 102his connected via an oil conduit 410 with ports 114c and 114g of the cutback valve 14, while the port 102i is connected via an oil conduit 411with ports 114a and 114d of the cut back valve 14 and also with ports112c and 112e of the pressure modifier valve 12.

The regulator valve 2 regulates the line pressure in the oil conduit 402in the following manner. Oil pressure from the port 102b acts on adifferential area between the lands 202a and 202b of the spool 202,applying a downward force, as viewed in FIG. 2(b), to the spool 202. Onthe other hand, the spool 202 is subject to the sum of an upward forceby the spring 302 and another upward force (which is later described) bythe spool 203. Since the oil within the port 102e which the port 102b isconnected with is adapted to be discharged to the port 102f, the oilpressure in the port 102b is regulated to a value at which the downwardforce created by the oil pressure in the port 102b balances with the sumof the two upward forces. If, as a result of an increase in the oilpressure in the port 102b, the downward force increases above the sum ofthe upward forces, the spool 202 slightly moves downward, forming aclearance between the land 202d and the port 102f, allowing the oil inthe port 102e to flow via this clearance to the port 102f resulting in areduction in the oil pressure in the port 102e which in turn results ina reduction in the oil pressure in the port 102b which communicates viathe oil conduit 402 with the port 102e. The reduction in the oilpressure in the port 102b causes a reduction in the downward force,allowing the spool 202 to move back upward. After this action isrepeated continuously, the oil pressure in the port 102b, i.e., the oilpressure in the oil conduit 402 is regulated to a value at which thedownward force always balances with the sum of the upward forces. Theoil pressure regulated in this manner (i.e., the line pressure) isvariable with the upward force by the spool 203 because the force due tothe spring 302 is constant. As shown, the ports 102h and 102i of thevalve bore 102 register with the ports 252a and 252b of the sleeve 252,respectively. This permits an oil pressure in the oil conduit 410 actingon a differential area between the lands 203a and 203b of the spool 203and an oil pressure in the oil conduit 411 to act on the bottom end ofthe land 203b, creating the above mentioned upward force with which thespool 203 urges the spool 202 upward. Therefore, the line pressure isregulated in response to the oil pressure in the oil conduit 410 andthat in the oil conduit 411. The actual characteristic of the linepressure will be described later after the description of the associatedvalves is completed.

Referring to FIG. 2(c), the manual valve 4 comprises a valve bore 104formed with six ports 104a to 104f and a spool 204 having formed thereontwo lands 204a and 204b slidably disposed in the valve bore 104. Thespool 204 is actuated by a shift lever disposed near a driver's seat,both not being illustrated, and is movable between six detent positions,i.e., a parking position "P", a reverse drive position "R", a neutralposition "N", a forward automatic drive position "D", a manual secondposition "II", and a manual first position "I". The port 104a is a drainport, while the port 104b is connected via an oil conduit 408 with theclutch C1 at an fluid operated actuator therein, not shown, and alsowith a port 502a of a shuttle valve 502. The port 104c is connected withthe before mentioned oil conduit 402 serving as the line pressurecircuit. The port 104d is connected via an oil conduit 412 with a port120g of the 1-2 shift valve 20, a port 124h of the 3-4 shift valve 24,and the governor valve 18. The port 104e is connected via an oil conduit414 with a port 116e of the line pressure booster valve 16, a port 122aof the 2-3 shift valve 22 and a port 504a of a shuttle valve 504, bothbeing shown in FIG. 2(b). The port 104f is connected via an oil conduit416 with a port 134d of the manual first range pressure reducing valve34. As shown in the following Table, the port 104c which is suppliedwith the line pressure is allowed to communicate with a port or portsindicated by the sign "o" in response to a position selected by thespool 204. The other port or ports which are not indicated by the sign"o" are drained.

                  TABLE                                                           ______________________________________                                                   Port                                                               Spool Range  104b   104d       104e 104f                                      ______________________________________                                        R            o                                                                N                                                                             D                   o                                                         H                   o          o                                              F                   o          o    o                                         ______________________________________                                    

Referring to FIG. 2(c), the throttle valve 6 comprises a valve bore 106formed with six ports 106a to 106f and a spool 206 having formed thereonthree lands 206a to 206c slidably disposed in the valve bore 106. Aspring 306 is disposed in the valve bore 106 to bias the spool 206leftward as viewed in FIG. 2(c). Also slidably disposed in the valvebore 106 is a plunger 207 operatively connected, via a linkage with anaccelerator pedal. Between the plunger 207 and the spool 206 is arrangeda spring 307. The land 206c is smaller in diameter than the other twolands 206a and 206b. The ports 106a and 106f are drain ports. The port106b is connected with an oil conduit 418, i.e., a kickdown pressurecircuit, to receive a kickdown pressure (the same pressure as the linepressure) only when the accelerator pedal is depressed to a kickdownposition thereof which will be described later. When the acceleratorpedal is in the other positions, this port 106b is connected with thedrain port 106a. The ports 106c and 106e are connected with an oilconduit 420, i.e., a throttle pressure circuit, and the port 106d isconnected with the oil conduit 402, serving as the line pressurecircuit. An orifice 610 is provided in the inlet to the port 106e.

With the throttle valve 6 thus constructed, an oil pressure in the ports106c and 106e is obtained after a well known pressure regulation iscarried out using the line pressure in the port 106d as a source ofpressure until the sum of a leftward force by the spring 306 and aleftward force created by the oil pressure in the port 106e acting onthe differential area between the lands 206b and 206c balances with arightward force by the spring 307. Thus, the oil pressure proportionalto the bias force of the spring 307 builds up in the oil conduit 420.However, since the bias force of the spring 307 is varied by the plunger207 operated by the accelerator pedal, the oil pressure, i.e., athrottle pressure, in the oil conduit 420 is proportional to thedepression degree of the accelerator pedal or the throttle openingdegree of the throttle valve mounted in an intake passage of the engine.The characteristic of the throttle pressure is illustrated by the solidline drawn curve shown in FIG. 3. Under kickdown condition, the throttlevalve 6 is pushed inwardly by the plunger 207 via the spring 307 to theright as viewed in FIG. 2(c) to the upper half position thereof so thatit ceases its pressure regulating function. Under this condition, theline pressure is supplied also to the oil conduit 420 and past a port108e to the oil conduit 418 and to the port 106b which served as a drainport.

The throttle fail safe valve 8 comprises a valve bore 108 connected withand formed coaxially with the valve bore 106 of the throttle valve 6.The valve bore 108 is formed with five ports 108a to 108e and slidablyreceives therein a sleeve 208 having formed thereon three lands 208a to208c. The throttle fail safe valve 8 also comprises the before mentionedplunger 207, the land 207a of which is slidably disposed in the sleeve208 for axial movement, a plug 209 closing the open end of valve bore108, and a spring 308 arranged between the plug 209 and the sleeve 208.The end plug 209 is formed with an aperture which allows the plunger 207to pass through. The ports 108a and 108d of the valve bore 108 areconnected with the oil conduit 420 which serves as the throttle pressurecircuit, and the port 108b is connected with the oil conduit 402 whichserves as the line pressure circuit. The port 108c is connected via theoil conduit 422 with a port 506a of a shuttle valve 506. The port 108eis connected with the oil conduit 418, i.e., the before mentionedkickdown circuit.

The throttle fail safe valve 8 thus constructed functions to alleviatethe force required to push the plunger 207 inwardly, so the acceleratorpedal is prevented from being excessively loaded by the spring 307 whenthe pedal is depressed. The throttle pressure supplied to the port 108aenters into the inside of the sleeve 208, acting on the land 207a of theplunger 207, thus applying to the plunger 207 the rightward force whichassists the depressing movement of the plunger 207. Thus, the forcerequired to push the plunger 207 by compressing the spring 307 isalleviated, alleviating the depressing force of the accelerator pedal.Irrespective of the fact that the reaction force of the spring 307increases as the spring 307 is compressed, substantially constantaccelerator pedal depression force is maintained because the throttlepressure increases as the spring 307 is compressed and the force derivedfrom the throttle pressure to push the plunger 207 also increases. Underthe kickdown condition, the plunger 207 is pushed inwardly to theposition illustrated by the upper half thereof as viewed in FIG. 2(c),allowing port 108e to communicate with the port 108a via the inside ofthe sleeve 208, thus allowing the oil pressure in the oil conduit 420 tobe supplied to the oil coonduit 418. Because, under the kickdowncondition, the spool 206 of the throttle valve 6 is also pushed inwardlyto the position illustrated by the upper half position thereof whereinthe port 106d is allowed to communicate with the port 106c, the oilconduit 420 which normally serves as the throttle pressure circuit andhas been supplied with the throttle pressure, is now supplied with theline pressure. Consequently, the line pressure develops also in the oilconduit 418. Since the sleeve 208 is biased by the spring 308 to stay inthe rightward limit position thereof as illustrated by the upper halfposition thereof wherein ports 108c and 108d are allowed to communicatewith each other via a groove between the lands 208a and 208b of thesleeve 208, the oil pressure in the oil conduit 420 is admitted to theoil conduit 422. Normally, the throttle pressure develops in the oilconduit, but the line pressure develops under the kickdown condition. Ifa throttle valve operating cable interconnecting the accelerator pedalwith the plunger 207 should be disconnected or broken, the plunger 207would be pulled outwardly by a return spring, not shown, to moveleftwardly as viewed in FIG. 2(c). This leftward movement of the plunger207 causes the sleeve 208 to move to the position indicated by the lowerhalf thereof viewing in FIG. 2(c). Thus, the port 108b communicates withthe port 108c, supplying the line pressure to the oil conduit 422 fromthe oil conduit 402, causing the line pressure to rise to a peak valuethereof, which will be described later. Therefore, the transmission isprevented from becoming burned or damaged due to low line pressure inthe event the throttle valve operating cable is disconnected or broken.The automotive vehicle is therefore enabled to travel (such as to arepair shop) without causing a slip in clutches and brakes. The throttlevalve operating mechanism employed here is disclosed and illustrated inFIG. 4 of U.S. Pat. No. 4,301,697 issued on Nov. 24, 1981 to KazuyoshiIwanaga et al. and assigned to Nissan Motor Company, Limited.

Referring to FIG. 2(c), the throttle modulator valve 10 comprises avalve bore 110 formed with five ports 110a to 110e, a spool 210 havingformed thereon three lands 210a to 210c slidably disposed in the valvebore 110, and a spring 310 biasing the spool 210 leftwardly as viewed inFIG. 2(c). The land 210a is larger in diameter than the diameter of thelands 210b and 210c. The ports 110a and 110d are connected with the oilconduit 420 which serves as the throttle pressure circuit, while theport 110b is connected with the oil conduit 418 which serves as thekickdown circuit. The ports 110c and 110e are connected via an oilconduit 424 with a port 122d of the 2-3 shift valve 22 shown in FIG.2(b). The port 110e is provided with an orifice 612 at an inlet thereof.

With the throttle modulator valve 10 thus constructed, since, when notunder the kickdown condition, the port 110b is a drain port via an oilconduit 418, a pressure regulation is effected with the port 110d (whichis supplied with the throttle pressure) as a high pressure side port andthe port 110b as a drain port. The equilibrium state of the spool 210 isaccomplished when a rightwardly directed force due to the oil pressure(throttle pressure) in the port 110a acting on the land 210a balancewith the sum of a leftwardly directed force derived from an oil pressurein the port 110e acting upon the land 210c and a leftwardly directedforce due to the spring 310. Therefore, the oil pressure in the port110e (this oil pressure hereinafter being called as "throttle modulatorpressure") varies depending upon the throttle pressure and reveals acharacteristic as shown in FIG. 3. This throttle modulator pressure issent via the above mentioned oil conduit 424 to the 2-3 shift valve 22to control same. Under the kickdown condition, the port 110b which hasbeen a drain port is supplied with the kickdown pressure (line pressure)and thus the throttle modulator valve 10 is moved to the upper halfposition thereof and ceases its pressure regulating function, allowingthe line pressure to appear in the oil conduit 424.

The pressure modifier valve 12 comprises a valve bore 112 formed withfive ports 112a to 112e, a spool 212 having formed thereon the samediameter lands 212a and 212b slidably disposed in the valve 112, and aspring 312 which biases the spool 212 downward viewing in FIG. 2(c). Theports 112a and 112b are both drain ports. The ports 112c and 112e areconnected via the above mentioned oil conduit 411 with the port 102i ofthe regulator valve 2, while the port 112d is connected via the oilconduit 426 with a port 506c of a shuttle valve 506. The port 112e isprovided with an orifice 622 at an inlet thereof. With the pressuremodifier valve 12 thus constructed, a pressure regulating function iseffected with the port 112d (which is supplied with the throttlepressure or the line pressure as will be later described) as a highpressure port and with the port 112b as a drain port. The equilibriumstate of the spool 212 is accomplished when an upward force, as viewedin FIG. 2(c), derived from an oil pressure in the port 112e acting onthe land 212b balances with a downward force due to the spring 312.Thus, the oil pressure in the port 112e (this pressure being hereinaftercalled as "pressure modifier pressure") has a constant value which isdetermined by the force of the spring 312. However, when the oilpressure supplied to the port 112d is low so that the upward force doesnot overcome the force by the spring 312, the pressure modifier valve 12stays in the right half position thereof wherein the pressure regulatingfunction is not effected, admitting the oil pressure in the port 112d tothe oil conduit 411. Thus, the oil pressure in the oil conduit 411becomes equal to the oil pressure in the oil conduit 426. Normally i.e.,when the throttle valve operating cable is disconnected or broken orwhen the manual valve 4 is not placed in the "II" position, the throttlepressure is supplied via the port 108c of the throttle fail safe valve8, the oil conduit 422, the shuttle valve 506 and the oil conduit 426 tothe port 112d, thus providing the pressure modifier pressure whichexibits a characteristic as shown in FIG. 4. If the throttle valveoperating cable should be disconnected or broken, the throttle fail safevalve 8 operates in the manner described before, allowing the linepressure to be supplied to the oil conduit 422, thus keeping thepressure modifier pressure constant. As will be later described, if themanual valve 4 is moved from the "D" position to "II" position, sincethe line pressure is supplied from the line pressure booster valve 16via the oil conduit 428 to the port 506b of the shuttle valve 506, theline pressure is supplied to the port 112d irrespective of the value ofthe throttle pressure within the oil conduit 422, thus allowing thepressure modifier pressure to increase to the constant value. Since thepressure modifier pressure is introduced to the port 102i of theregulator valve 2 via the oil conduit 411, the line pressure increasesin accordance with the magnitude of the pressure modifier pressure. Thepressure modifier pressure is fed also to the cut back valve 14.

The cut back valve 14 comprises a valve bore 114 formed with seven ports114a to 114g and a spool 256 having formed thereon three lands 214a to214c slidably disposed in the valve bore 114. It also comprises a sleeve256 formed with ports 256a and 256b in registry respectively with theports 114f and 114g, and a spool 215 having formed thereon two lands215a and 215b slidably disposed in the sleeve 256. The land 214a of thespool 214 is smaller in diameter than the lands 214b and 241c. The land215a of the spool 215 is larger in diameter than the land 215b. Theports 114a and 114d are connected with the oil conduit 411 and aresupplied with the before mentioned pressure modifier pressure. The port114b is a drain port. The ports 114c and 114g are connected with an oilconduit 410, the port 114e is connected with the oil conduit 428, andthe port 114f is connected with an oil conduit 430 which serves as agovernor pressure circuit. The port 114g is provided with an orifice 614at an inlet thereof. The cut back valve 14 thus constructed performs apressure regulating function with the port 114d as a high pressure portand with the port 114b as a drain port when the manual valve 4 is notplaced in "II" position or the line pressure booster valve 16 is in theupper half position thereof as viewed in FIG. 2(c) (i.e., when the oilconduit 428 is a drain oil conduit). Under this condition, the spools214 and 215 move as if they were one piece and assume an equilibriumstate when a rightwardly directed force derived from the pressuremodifier pressure in the port 114a acting on the land 214a balances withthe sum of a leftwardly directed force derived from an oil pressure inthe port 114g acting on the land 215b and a leftwardly directed forcederived from an oil pressure (governor pressure) in the port 114f actingon a differential area between the lands 215a and 215b. As the governorpressure rises, the oil pressure in the port 114g (which oil pressure ishereinafter called as " cut back pressure") drops and when the governorpressure rises further beyond a predetermined value, the cut back valve14 is moved to the lower half position thereof as viewed in FIG. 2(c),reducing the cut back pressure to zero. The cut back pressure drops asthe pressure modifier pressure drops. The characteristic of this cutback pressure is illustrated in FIG. 5. Since the cut back pressure isintroduced to the port 102h of the regulator valve 2 via the oil conduit410, the line pressure drops as the governor pressure rises and it dropsas the pressure modifier pressure drops. If the manual valve 4 is movedfrom the "D" position to the "II" position, the line pressure issupplied to the port 114e of the cut back valve 14 via the oil conduit428, thus urging the spool 214 to move to the leftward limit position asviewed in FIG. 2(c). Consequently, the port 114c is allowed tocommunicate with the port 114b which is a drain port, so the oilpressure in the oil conduit 410 drops to zero irrespective of the valueof the governor pressure nor that of the pressure modifier pressure.

The line pressure booster valve 16 comprises a valve bore 116 formedwith six ports 116a to 116f, a spool having formed thereon three lands216a to 216c slidably disposed in the valve bore 116, and a spring 316biasing the spool 216 leftwardly as viewed in FIG. 2(c). The lands 126ato 216c are of the same diameter land. The spool 216 is formed with abore 216d which communicates with a groove between the lands 216a and216b with the left end of the spool 216. The ports 116c and 116f aredrain ports. The port 116d is connected with the oil conduit 428, andthe port 116e is connected with the oil conduit 414. The port 116b isconnnected via the oil conduit 432 with a port 120h of the 1-2 shiftvalve 20 shown in FIG. 2(b), which port 120h is adapted to be suppliedwith the line pressure when the 1-2 shift valve 20 is in the secondspeed ratio position (an upshift position thereof). The port 116a isconnected via an oil conduit 434 with a port 122g of the 2-3 shift valve22, which port 122g is adapted to be supplied with the line pressurewhen the 2-3 shift valve 22 is in the third speed ratio position (anupshift position thereof). The line pressure booster valve 16 thusconstructed operates in a different manner in a case where the 2-3 shiftvalve 22 is moved from the second speed ratio position (the downshiftposition thereof) to the third speed ratio position from a case whereinit is moved from the third speed ratio position to the second speedratio position. That is, when the 1-2 shift valve 20 is in the firstspeed ratio position or the second speed ratio position, the spool 216of the line pressure booster valve 16 stays in the upper half positionthereof as viewed in FIG. 2(c) by the action of the spring 316. When the1-2 shift valve 20 is in the second speed ratio position, the linepressure is supplied to the port 116b. This, however, does not cause theline pressure booster valve 16 to shift its position because the land216b blocks the port 116b. In this state, the port 116d communicateswith the drain port 116c. When, subsequently, the 2-3 shift valve 22 ismoved to the third speed ratio position, the line pressure is suppliedto the oil conduit 434. The line pressure in the oil conduit 434 passesthrough the port 116a and the bore 216d of the spool 216 to act on theleft end face of the spool 216, urging the spool 216 to move against thespring 316 rightwardly to the lower half position thereof as viewed inFIG. 2(c). In this position of the spool 216, the port 116b is allowedto communicate with the bore 216d of the spool 216, while the port 116ais blocked by the land 216a, so that the pressure in the oil conduit 432now acts on the left end face of the spool 216. Thus, the line pressurebooster valve 16 stays in the lower half position thereof even after the2-3 shift valve 22 subsequently returns to the second speed ratioposition thereof. Even if the line pressure booster valve 16 is moved tothe lower half position thereof, the oil conduit 428 continues to bedrained via the port 116d, port 116e and oil conduit 414 to the port104e as long as the manual valve 4 is not in the "II" or "I" positionwherein the port 104e of the manual valve 4 is a drain port. However,when the manual valve 4 is placed in the "II " or "I" position, sincethe line pressure is supplied to the oil conduit 414, the line pressureis supplied via the ports 116e and 116d to the oil conduit 428. The linepressure in the oil conduit 428 is supplied via the shuttle valve 506 tothe port 112d of the pressure modifier valve 12, rendering the pressuremodifier valve 12 to boost the line pressure. Therefore, in the casewhere the manual valve 4 is moved to the "II" position or "I" positionwhile the vehicle is travelling in the third speed ratio in order toeffect a forced downshift to the second speed ratio to obtain effectiveengine braking, the line pressure is boosted high enough to provide theswift downshift and effective engine braking. If the 1-2 shift valve 20is moved to the first speed ratio position, since the oil pressure inthe oil conduit 432 disappears, the line pressure booster valve 16returns to the upper half position thereof, ceasing its function toboost the line pressure. The governor valve 18 is installed to rotatewith the output shaft O of the automatic transmission and effectspressure regulation using the line pressure (i.e., the line pressurebeing fed to the governor valve 18 when the manual valve 4 is in the "D"position, "II" position or "I" position) so as to supply an oil pressure(governor pressure) variable with the vehicle speed. The governorpressure reveals a characteristic as shown in FIG. 6.

Referring to FIG. 2(b), the 1-2 shift valve 20 comprises a valve bore120 formed with eleven ports 120a to 120k, two spools 220 and 221slidably disposed in the valve bore 120, and a spring 320 biasing thespool 220 downward as viewed in FIG. 2(b). The spool 220 is formed withthree lands 220a to 220c which are larger in diameter in this sequence,while the spool 221 is formed with lands 221a to 221d (the lands 221a to221c are the same in diameter, but the land 221d is larger in diameterthan the former three). The ports 120a, 120f and 120i are drain ports.The port 120b is connected with the oil conduit 418 serving as thekickdown pressure circuit. The port 120b is arranged such that the oilpressure therein acts on a differential area between the lands 220a and220b when the spool 220 is in the down position thereof indicated by theright half position thereof viewing in FIG. 2(b), urging the spool 220downward, and acts on a differential area between the lands 220a and220c when the spool 220 is in the up position thereof indicated by theleft half position thereof viewing in FIG. 2(b), urging the spool 220downward. The port 120c is connected with the oil conduit 420 serving asthe throttle pressure circuit. The port 120c is arranged such that theoil pressure therein acts on a differential area between the lands 220band 220c when the spool 220 is in the down position thereof so as tourge the spool 220 downward, but no downward force is provided when thespool 220 is in the up position thereof because the oil pressure acts onthe circumference of the land 220c. The ports 120j and 120k areconnected with the oil conduit 430 which serves as the governor pressurecircuit. These ports are arranged such that the governor pressure actson an area (which is equal to the area of the land 221c) resulting fromsubtracting a differential area between the lands 221d and 221c from thearea of the land 221d when the spool 221 is in the downshift positionthereof, urging the spool 221 upwardly, and acts on the area of the land221d when the spool 221 is in the upshift position thereof, urging thespool 221 upward. The port 120g which is connected with the oil conduit412 is blocked by the land 221b when the spool 221 is in the downshiftposition thereof, while it communicates via the port 120h with the oilconduit 432 when the spool 221 is in the upshift position thereof. Theoil conduit 432 is connected with the servo apply chamber S/A of thebrake B2 shown in FIG. 2(a). The port 120d is connected via the oilconduit 436 with the port 502c of the shuttle valve 502 shown in FIG.2(c). The port 102d is adapted to communicate with the port 120e whenthe spool 221 is in the downshift position thereof. The port 120e isconnected via an oil conduit 438 with the brake B1 shown in FIG. 2(a).With the 1-3 shift valve 20 thus constructed, the supply of thepressurized oil to the servo apply chamber S/A and the brake B1 iscontrolled depending upon the downshift position of the spool 221 orupshift position thereof, the further detail being described later.

The 2-3 shift valve 22 comprises a valve bore 122 formed with ten ports122a to 122j, a spool 22 having formed thereon five lands 222a to 222eslidably disposed in the valve bore 122, a plug 223 axially slidablenear the closed end in the valve bore 122, and a spring 322 arrangedbetween the spool 222 and the plug 223. The lands 222a to 222c arelarger in diameter in this sequence, the land 222c is the same indiameter as the land 222d, and the land 222e is larger in diameter thanthe former two. The ports 122b and 122h are drain ports. The port 122ais connected with the oil conduit 414 (which is supplied with the linepressure when the manual valve 4 is in the "II" position or "I"position), and is arranged such that the oil pressure therein acts onthe upper end face of the plug 223. The port 122c is connected with theoil conduit 418 serving as the kickdown pressure circuit and is arrangedsuch that the oil pressure in the port 122c acts on a differential areabetween the lands 222a and 222b of the spool 222, urging the spool 222downward as viewed in FIG. 2(b). The port 122d is supplied with athrottle modulator pressure from the oil conduit 424 and is arrangedsuch that the throttle modulator pressure acts on a differential areabetween the lands 222b and 222c when the spool 222 is in the upshiftposition thereof, urging the spool 222 downward as viewed in FIG. 2(b).The port 122e is supplied with a throttle pressure from the oil conduit420 and is arranged such that the throttle pressure acts on adifferential area between the lands 222b and 222c of the spool 222 whenthe spool 222 is in a downshift position thereof, urging the spool 222downward. The ports 122i and 122j are supplied with the governorpressure from the oil conduit 430, and they are arranged such that thegovernor pressure acts on an area (i.e., the area of the land 222d)resulting from subtracting a differential area between the lands 222eand 222d from the area of the land 222e when the spool 222 is in thedownshift position thereof, urging the spool 222 upward, while it actson the area of the land 222e when the spool 222 is in the upshiftposition thereof, urging the spool 222 upward. The port 122f providedwith an orifice 616 at its inlet is connected with the oil conduit 432,and the port 122g is connected with the oil conduit 434. These ports122f and 122g are arranged such that they communicate with each otherwhen the spool 222 is in the upshift position thereof. The oil conduit434 is connected with the clutch C2 shown in FIG. 2(a). With the 2-3shift valve 22 thus constructed, the supply of pressurized oil to theclutch C2 is controlled depending upon the downshift or upshift positionof the spool 222, the further detail being described later.

The 3-3 shift valve 24 comprises a valve bore 124 formed with elevenports 124a to 124k, a spool 224 having formed thereon four lands 224a to224d slidably disposed in the valve bore 124, a plug 225 axiallyslidable in the valve bore 124 near the closed end thereof, and a spring324 arranged between the spool 224 and the plug 225. The lands 224a to224c are the same in diameter and the land 224d is larger in diameterthan the former lands. The port 124a is connected with the oil conduit420 which serves as the throttle pressure circuit and is arranged suchthat the oil pressure in the port 124a acts on the end face of the plug225 so as to urge the plug 225 downward as viewed in FIG. 2(b). The port124b is connected via an oil conduit 440 with a port 508c of a shuttlevalve 508. The oil pressure in this port 124b always acts on the upperside of the land 224a so as to urge the spool 224 downwardly. The port124c is connected via an oil conduit 442 with the clutch C3 shown inFIG. 2(a). This port 124c is allowed to communicate with the port 124dwhich is connected via the oil conduit 444 with the servo releasechamber S/R of the brake B2 shown in FIG. 2(a) when the spool 224 is inthe upshift position thereof. When the spool 224 is in the downshiftposition thereof, the port 124d communicates with the port 124e which isconnected via the oil conduit 434 with the clutch C2. The port 124f isconnected via an oil conduit 446 with a port 126d of the 2-4 timingvalve 26, while the port 124g is connected with the oil conduit 442. Theports 124f and 124g are allowed to communicate with each other when thespool 224 is in the upshift position thereof. The port 124g, on otherhand, is allowed to communicate with the port 124h which is connectedwith the oil conduit 412 when the spool 224 is in the downshift positionthereof. The port 124i is a drain port. The ports 124j and 124k areconnected with the oil conduit 430 which serves as the governor pressurecircuit. Similarly to the before mentioned 1-2 shift valve 20 and 2-3shift valve 22, the ports 124j and 124k are arranged such that thegovernor pressure acts on the area of the land 224c when the spool 224is in the downshift position thereof, urging the spool 224 upward, whilewhen the spool 224 is in the upshift position thereof, the governorpressure acts on the area of the land 224d, urging the spool 224 upward.With the 3-4 shift valve 24 thus constructed, the supply of pressurizedoil to the clutch C3 and the servo release chamber S/R is controlleddepending upon the downshift position or upshift position of the spool224.

The 2-4 timing valve 26 comprises a valve bore 126 formed with six ports126a to 126f and a spool 226 having formed thereon three lands 226a to226c slidably disposed in the valve bore 126. The lands 226b and 226care the same in diameter, and the land 226a is smaller in diameter thanthe former two. The ports 126a, 126c, and 126e are drain ports,respectively. The port 126e is provided with an orifice 602. An orifice618 is provided in an oil conduit 446. The port 126b is supplied withthe throttle pressure from the oil conduit 420 which serve as thethrottle pressure circuit and is arranged such that the throttlepressure in the port 126b always acts on a differential area between thelands 226a and 226b so as to always urge the spool 226 downward. Theport 126d is connected with the oil conduit 446 and is allowed tocommunicate with the port 126e when the spool 226 is in the downposition thereof, but is allowed to communicate with the port 126c whenthe spool 226 is in the up position thereof. The port 126f is connectedvia the oil conduit 434 with the clutch C2. With the 2-4 timing valve 26thus constructed, the pressurized oil is supplied to the clutch C2 andthe pressurized oil from the clutch C3 is discharged at proper timingduring 2-4 shift, further detail being described later.

Referring to FIG. 2(a), the 2-3 timing valve 28 comprises a valve bore128 formed with five ports 128a to 128e, a spool 228 having formedthereon three lands 228a to 228c slidably disposed in the valve bore128, and a spring 328 biasing the spool 228 upwardly as viewed in FIG.2(a). The port 128a is supplied with the governor pressure from the oilconduit 430 so as to urge the spool 228 downward. As opposed to thisforce, the throttle pressure is supplied to the port 128e from the oilconduit 420 so as to urge the spool 228 upward. The port 128b is a drainport. The port 128c is connected with a portion of the oil conduit 434upstream of the check valve 750 and an orifice 650 (they are arranged inparallel), i.e., which portion is disposed on the side near the 2-3shift valve 22, while the port 128d is connected with a portion of theoil conduit 434 downstream of the check valve 750 and the orifice 650 (aportion on the side near the clutch C2). With the 2-3 timing valve 28thus constructed, the pressurized oil to be supplied to the clutch C2 iscontrolled depending upon the magnitude of the throttle pressure andthat of the governor pressure so that the clutch C2 is engaged at propertiming during 2-3 shift.

The 3-4 timing valve 30 comprises a valve bore 130 formed with fiveports 130a to 130e, a spool 230 having formed thereon three lands 230ato 230c slidably disposed in the valve bore 130, and a spring 330biasing the spool 230 upward as viewed in FIG. 2(a). The port 130a issupplied with the governor pressure from the oil conduit 430 so as tourge the spool 230 downward. As opposed to this force, the throttlepressure is supplied to the port 130e from the oil conduit 420 so as tourge the spool 230 upward. The port 130b is a drain port. The ports 130cand 130d are both connected with the oil conduit 442. The port 130c isconnected with a portion of the oil conduit 442 upstream of a checkvalve 752 and an orifice 652 (they are arranged in parallel), i.e.,which portion is disposed on the side near the port 124c of the 3-4shift valve 22, while the port 130d is connected with the downstreamportion which is disposed on the side near the clutch C3. With the 3-4timing valve 30 thus constructed, the pressurized oil to be dischargedfrom the clutch C3 is controlled depending upon the magnitude of thethrottle pressure and that of the governor pressure so as to release theclutch C3 in proper timing during 3-4 shift.

The 3-2 timing valve 32 comprises a valve bore 132 formed with fiveports 132a to 132e, a spool 232 having formed thereon three lands 232ato 232c slidably disposed in the valve bore 132, and a spring 332biasing the spool 232 upward as viewed in FIG. 2(a). The port 132a issupplied with the governor pressure from the oil conduit 430 so as tourge the spool 232 downward. On the other hand, as opposed to thisforce, the throttle pressure is supplied to the port 132e from the oilconduit 420 so as to urge the spool 230 upward. The port 132b is a drainport. The ports 132c and 132d are both connected with the oil conduit434. The port 132c is connected with a portion of the oil conduit 434upstream of a check valve 754 and an orifice 654 (they are arranged inparallel) which portion is disposed on the side directly connected withthe port 122g of the 2-3 shift valve 22, while, the port 132d isconnected with the downstream portion of the oil conduit 434 which isdisposed on the side directly connected with the port 124e of the 3-4shift valve 24. With the 3-2 timing valve 32 thus constructed, thepressurized oil to be discharged from the clutch C2 is controlleddepending upon the magnitude of the throttle pressure and that of thegovernor pressure so as to release the clutch C2 at proper timing during3-2 shift.

Referring to FIG. 2(c), the manual first range pressure reducing valve34 comprises a valve bore 134 formed with five ports 134a to 134e, aspool 234 having formed thereon two lands 234a and 234b, and a spring334 biasing the spool 234 downward. The ports 134a and 134b are drainports. The port 134d is connected with the oil conduit 416 which issupplied with the line pressure when the manual valve 4 is in the "I"position, while the ports 134c and 134e are connected with the oilconduit 448. The port 134e is provided with an orifice 620 at an inletthereof. With the manual first range pressure reducing valve 34, apressure regulation is effected with the port 134d as a high pressureport and with the port 134b as a drain port until the oil pressure inthe port 134e balances with the force of the spring 334. Thus, themanual first range pressure reducing valve 34 functions to produce apredetermined constant oil pressure in the oil conduit 448 which resultsfrom reducing the line pressure when the line pressure appears in theoil conduit 416.

Referring to FIG. 2(a), the torque converter pressure reducing valve 36comprises a valve bore 136 formed with five ports 136a to 136e, a spool236 having formed thereon two lands 236a and 236b, and a spring 336biasing the spool 236 leftwardly as viewed in FIG. 2(a). The ports 136dand 136e are drain ports. The port 136b is connected with the oilconduit 406 which is supplied with pressurized oil discharged from theport 102f of the regulator valve 2, while the ports 136a and 136c areconnected via the oil conduit 450 with the torque converter T/C. Theport 136a is provided with an orifice 624 at an inlet thereof. With thetorque converter pressure reducing valve 36 thus constructed, a pressureregulation is effected with the port 136d as a high pressure port andwith the port 136b as a drain port until the force created by the oilpressure in the port 136a balances with the force of the spring 336.Therefore, the torque converter pressure reducing valve 36 functions toprovide a constant pressurized pressure to the torque converter T/Cirrespective of the variation in oil pressure in the oil conduit 406.

The 1-2 accumulator 38 comprises a stepped cylindrical bore 138, apiston 238 slidably disposed in the bore 138, and a spring 338 biasingthe piston upwardly as viewed in FIG. 2(a). A chamber 138a defined bythe larger diameter side of the piston 238 is connected with the oilconduit 432, a chamber 138b defined by the smaller diameter side of thepiston 238 is connected with the oil conduit 402 (line pressurecircuit), and an intermediate chamber 138c is a drain chamber. Anorifice 656 and a check valve 756 are provided in parallel at thatportion of the oil conduit 432 which is disposed upstream of the 1-2accumulator 38 and the servo apply chamber S/A. The 1-2 accumulator 38thus constructed functions to allow a gradual rise in the oil pressurein the oil conduit 432 (i.e., the oil pressure in the servo applychamber S/A) so as to effect a smooth 1-2 shift.

The 4-3 accumulator 40 comprises a cylindrical bore 140, a piston 240slidably disposed in the cylindrical bore 140, and a spring 340 biasingthe piston 240 upward as viewed in FIG. 2(a). An upper side chamber 140adefined by the piston 240 is connected with the oil conduit 402 (theline pressure circuit), while the lower side chamber 140b is connectedwith the oil conduit 442 which communicates with the clutch C3. A checkvalve 758 and an orifice 658 are arranged in parallel at a portion ofthe oil conduit 442 which is disposed upstream of the clutch C3 and the4-3 accumulator 40. The 4-3 accumulator 40 thus constructed functions toallow a gradual rise in the oil pressure in the oil conduit 442 (the oilpressure in the clutch C3) so as to effect a smooth 4-3 shift and so asto alleviate a shock upon moving the manual valve 4 from the "N"position to the "D" position.

Referring to FIG. 2(a), the overdrive inhibitor solenoid 42 is providedto face an opening 409a formed in an oil conduit 409 which is connectedat one end thereof with the oil conduit 402 serving as the line pressurecircuit and it comprises a rod 42a arranged to close the opening 409awhen the solenoid 42 is energized. The oil conduit 409 is connected witha port 504b of a shuttle valve 504. The overdrive inhibitor solenoid 42is energized by the overdrive inhibitor switch SW operable from adriver's seat. When the switch SW is in the off state thereof, the oilflowing into the oil conduit 409 from the oil conduit 402 is dischargedfrom the opening 409a, so no pressure develops in the oil conduit 409.As will be noted, since the area of the orifice 604 is small enough, theline pressure in the oil conduit 402 is not affected even if the oil isdischarged from the opening 409a. When the switch SW is turned on andthe rod 42a closes the opening 409a, the oil conduit 409 is subject tothe same pressure, i.e., the line pressure, as that in the oil conduit402. As a result, the 3-4 shift valve 24 is held in the downshiftposition thereof as will be described later, thus preventing an upshiftto the fourth speed ratio (overdrive).

The clutches C1, C3, and the servo apply chamber S/A are provided withorifices 626, 628 and 630 at inlets thereof, respectively.

Hereinafter, the operation of the hydraulic control system is described.

The line pressure obtained by the pressure regulation in the regulatorvalve 2 is described.

As described before, the line pressure in the oil conduit 402 isdetermined by the sum of the upward forces applied to the spool 203.Thus, since the pressure modifier pressure from the pressure modifiervalve 12 and the cut back pressure from the cut back valve 14 act on thespool 203 to urge same upward, the line pressure is variable with thevariation of these pressures. The pressure modifier pressure and the cutback pressure have characteristics as shown in FIGS. 4 and 5,respectively. The line pressure actually obtained, therefore, showscharacteristic as shown in FIG. 7 in terms of the throttle openingdegree. When the vehicle travels with the manual valve 4 set in the "II"position or "I" position, since the pressure modifier pressure isconstant and the cut back pressure is zero, the line pressure continuesto be constant irrespective of the variation in the throttle pressureand that in the governor pressure. When the manual valve 4 is set in the"II" position or "I" position, the line pressure builds up in the oilconduit 428 due to the function of the line pressure booster valve 16,the line pressure in the oil conduit 428 switches the shuttle valve 506and reaches the oil conduit 426. Thus, the pressure modifier valve 12receives this line pressure at its port 112d, so it produces theconstant pressure in the oil conduit 411. The line pressure in the oilconduit 428 is supplied also to the port 114e of the cut back valve 14,urging the spool 214 to move leftwardly and the spool 215 to moverightwardly, allowing the oil conduit 410 to communicate with the drainport 114b. Thus, the cut back pressure in the oil conduit 410 drops tozero. Referring to FIG. 7, the reference character No represents therotational speed of the transmission output shaft O so that the linepressure varies with the variation in the vehicle speed. FIG. 8 showsthe variation in line pressure versus the rotational speed of the outputshaft O.

The operation of the manual valve 4 in each of the different positionsis described.

When the manual valve 4 is in the "N" position thereof, the linepressure is not supplied to none of the ports 104b, 104d, 104e and 104f,rendering them to serve as drain ports. Thus, no oil pressure issupplied to the clutches and brakes, conditioning the automatictransmission in neutral state.

When the manual valve 4 is moved from the "N" position to the "D"position, the line pressure is supplied to the port 104d from the port104c, and this line pressure in the port 104d is supplied to the port120g of the 1-2 shift valve 20, the port 124h of the 3-4 shift valve 24and the governor valve 18. The governor valve 18 generates the beforementioned governor pressure variable with the vehicle speed using theline pressure and provides the governor pressure via the oil conduit 430to the respective shift valves 20, 22 and 24. When the vehicle speed islow, since the governor pressure is low, each of the shift valves 20, 22and 24 stays in the downshift position thereof. Therefore, the linepressure supplied to the port 120g of the 1-2 shift valve 20 is blockedby the land 221b. On the other hand, since the port 124h of the 3-4shift valve 24 is allowed to communicate with the port 124g, the linepressure is supplied to the oil conduit 442, and the line pressure inthe oil conduit 442 passes through orifices 658 and 628 to reach theclutch C3 for engagement thereof. As described before, the engagement ofthe clutch C3 cooperates with the action of the one-way brake OWC toestablish the first forward speed ratio. Since the oil conduit 442communicates with the chamber 140b of the 4-3 accumulator 40, the linepressure is supplied to the chamber 140b, allowing the piston 240 of the4-3 accumulator 40 which has been pressed down by the line pressuresupplied from the oil conduit 402 to move slowly in the upward directionin response to the action of the spring 340. This causes the oilpressure downstream of the orifices 658 to rise slowly, causing gradualengagement of the clutch C3, thus alleviating a shock (a select shock)upon shifting from the "N" position to the "D" position.

When the governor pressure rises to a predetermined value as a resultthat the vehicle speed has increased after the automotive vehicle hasstarted with the first speed ratio, the upward force due to the governorpressure acting on the spools 220 and 221 overcomes the sum of thedownward force by the spring 320 and that due to the throttle pressureacting on the differential area between the lands 220b and 220c, thusallowing the spools 220 and 221 to begin to move upward from thedownshift position thereof. Immediately after the spools 220 and 221 hasbegun to move upward, the land 221d of the spool 221 closes the port120j at the same time the land 221c opens the drain port 120i, resultingin a rapid increase in pressure acting area which the governor pressureacts on, allowing the spools 220 and 221 to move to the illustrated lefthalf position thereof in a moment. As a result, the port 120gcommunicates with the port 120h, admitting the line pressure to the oilconduit 432. The line pressure in the oil conduit 432 is supplied viathe orifices 656 and 630 to the servo apply chamber S/A. Thus, the brakeB2 is engaged and cooperates with the clutch C3 which remains engaged toestablish the second forward speed ratio. The line pressure in the oilconduit 432 is admitted to the chamber 138a of the 1-2 accumulator 38,pushing the piston 238 which has been pressed downward by the linepressure acting in the chamber 138b from the oil conduit 402 back in theupward direction. This causes the oil pressure in the downstream portionof the oil conduit 432 to rise gradually, causing the gradual engagementof the brake B2. Consequently, a shock (a shift shock) during a shiftfrom the first speed ratio to the second speed ratio is alleviated.

When the governor pressure further rises up to another value, the upwardforce due to the governor pressure and acting on the spool 222 of the2-3 shift valve 22 overcomes the sum of the downward force due to thespring 322 and that due to the throttle pressure acting on thedifferential area between the lands 222b and 222c, allowing the spool222 to begin to move upward from the downshift position thereof.Immediately after the spool 222 has begun to move upward, the land 222ecloses the port 122i at the same time the land 222d opens the drain port122h, causing a rapid increase in the pressure acting area which thegovernor pressure acts on, and concurrently with this event, thethrottle pressure from the port 122e acting on the differential areabetween the lands 222b and 222c is replaced by the relatively lowthrottle modulator pressure, resulting in a reduction in the downwardforce. Therefore, the spool 222 moves to the illustrated left halfposition thereof in a moment. Thus, the port 122f is allowed tocommunicate with the port 122g, admitting the line pressure in the oilconduit 432 to the oil conduit 434. Since the oil conduit 434communicates with the clutch C2, the clutch C2 is engaged. The oilconduit 434 communicates with the port 124e of the 3-4 shift valve 24which in turn communicates with the port 124d when the 3-4 shift valve24 is in the downshift position thereof, so that the line pressure issupplied also to the port 124d. Thus, through the oil conduit 444connected with the port 124d, the line pressure is supplied to the servorelease chamber S/R of the brake B2, releasing the brake B2. As aresult, the clutch C2 is engaged and the brake B2 is released with theclutch C3 left engaged to establish the third forward speed ratio. If,during the shift from the second speed ratio to the third speed ratio,the engagement of the clutch C2 were not effected in the proper timingwith the release of the brake B2, a great shock would take place or theengine would race. In order to provide the proper timing, the 2-3 timingvalve 28 is arranged in the oil conduit 434 at a portion between the 2-3shift valve 22 and the clutch C2. The spool 228 of the 2-3 timing valve28 is moved to the illustrated right half position thereof or the lefthalf position thereof viewing in FIG. 2(a) depending upon the magnitudeof the downward force due to the governor pressure supplied to the port128a and the magnitude of the sum of the upward force due to the spring328 and that due to the throttle pressure supplied to the port 128e.That is, in an acceleration state when the accelerator pedal isdepressed grately, since the throttle pressure is high, the 2-3 timingvalve 28 assumes the illustrated right half position thereof wherein theports 128c and 128d communicate with each other, the upstream portion ofthe oil conduit 434 near the port 122g of the 2-3 shift valve 22 isconnected with the downstream portion of the oil conduit 434 near theclutch C2 bypassing the orifice 650, thus allowing the clutch C2 toengage swiftly. On the other hand, if the depression degree of theaccelerator pedal is decreased, the throttle pressure drops to allow the2-3 timing valve 28 to assume the illustrated left half positionthereof, shutting off the communication between the ports 128c and 128d,providing the state wherein the upstream portion of the oil conduit 434near the port 122g of the 2-3 shift valve 22 communicates with thedownstream portion of the oil conduit 434 near the clutch C2 through theorifice 650. Thus, the oil pressure in the clutch C2 increasesgradually, so the engagement of the clutch C2 is slightly delayed.During the period of this delay, the engine revolution speed drops, sothat the shift shock is alleviated correspondingly. Hence, the 2-3timing valve 28 functions to alleviate the shock taking place duringshift from the second speed ratio to the third speed ratio while thevehicle is coasting. Midway in the oil conduit 434 extending from theport 122g of the 2-3 shift valve 22 to the port 124e of the 3-4 shiftvalve 24, the 3-2 timing valve 32, check valve 754 and orifice 654 arearranged in parallel, but since the flow of oil from the port 122gtoward the port 124e agrees with the direction of flow which is allowedby the check valve 754, the oil pressure is supplied to the servorelease chamber S/R without being subject to any influence of the flowrestriction irrespective of the positions of the 3-2 timing valve 32.

When the governor pressure rises further to still another value as aresult that the vehicle has increased its vehicle speed duringtravelling with the third speed ratio, the upward force due to thegovernor pressure acting on the spool 224 of the 3-4 shift valve 34overcomes the downward force due to the throttle pressure from the port124a acting on the upper end face of the plug 225, causing the spool 224to begin to move upward from the downshift position thereof. When thethrottle pressure is low, since the force of the spring 324 is larger,the plug 225 is pushed upward by the spring 324 so that the abovementioned downward force is provided by the constant force by the spring324. Immediately after the spool 224 has begun to move upward, the land224d of the spool 224 closes the port 124j at the same time the land224c opens the drain port 124i, rapidly increasing the pressure actingarea which the governor pressure acts on, urging the spool 224 to moveupward to the illustrated left half position thereof in a moment. Inthis position of the spool 224, the port 124d connected with the servorelease chamber S/R is allowed to communicate with the port 124cconnected with the oil conduit 442, while the port 124g connected withthe oil conduit 442 is allowed to communicate with the port 124f.Therefore, both the clutch C3 and servo release chamber S/R are allowedto communicate with the port 124f. The port 124f is connected via theoil conduit 446 with the port 126d of the 2-4 timing valve 26, but sincewhen the 2-4 timing valve 26 is urged upward by the oil pressure actingthereon from the oil conduit 434 and assumes the illustrated right halfposition thereof, the port 126d communicates with the port 126c which isa drain port. As a result, the oil pressure in the clutch C3 and that inthe servo release chamber S/R are both discharged, disengaging theclutch C3 and engaging the brake B2. Since the clutch C2 is engaged andthe brake B2 is engaged, the fourth speed ratio is established. Duringthe shift to the fourth speed ratio, since the oil from the clutch C3flows through the check valve 758 in the forward direction thereof (theflow direction allowed by the check valve 758), the oil pressure in theclutch C3 is discharged swiftly. On the other hand, since the 3-4 timingvalve 30, check valve 752 and orifice 652 are arranged in parallel inthe midway in the oil conduit 442 extending from the port 124c to theport 124g, the speed at which the oil pressure is discharged from theservo release chamber S/R differs depending upon the positions assumedby the 3-4 timing valve 30. The 3-4 timing valve 30 is substantially thesame in construction as the before mentioned 2-3 timing valve 28. Duringacceleration, the 3-4 timing valve 30 allows the port 124c of the 3-4shift valve 24 to communicate with the port 124g of the 3-4 shift valve24 bypassing the orifice 652, while in coasting operation it shuts offthe communication between the port 124c and 124g. Thus, in the lattercase, the ports 124c and 124g are connected with each other through theorifice 652. As a result, in coasting operation the oil pressure in theservo release chamber S/R drops gradually and thus the engagement of thebrake B2 is delayed as compared to the disengagement of the clutch C3.During the short period of this delay, the engine revolution speeddrops, thus alleviating the shift shock correspondingly.

The operation of the 2-4 timing valve 26 is described. If, while thevehicle is travelling with the second speed ratio, the throttle pressureis decreased rapidly, there are occasions when the 2-3 shift valve 22and 3-4 shift valve 24 are moved from their downshift positions to theirupshift positions at the same time. This causes a shift from the secondspeed ratio directly to the fourth speed ratio. In this case, the clutchC3 is disengaged and the clutch C2 is engaged. For preventing theoccurrence of the shift shock or racing of the engine, it is necessaryto disengage the clutch C3 in the proper timing with the engagement ofthe clutch C2. The oil pressure in the clutch C2 is increased by the oilsupplied thereto from the 2-3 shift valve 22 via the oil conduit 434,and this oil pressure in the clutch C2 is also supplied to the port 126fof the 2-4 timing valve 26. The 2-4 timing valve 26 assumes theillustrated left half position thereof in the second speed ratio statebecause it is pressed down by the throttle pressure supplied to the port126b. In this position of the 2-4 timing valve 26, the ports 126d and126e are allowed to communicate with each other, so that the oilpressure in the clutch C3 is discharged via the oil conduit 442, theports 124g and 124f of the 3-4 shift valve 24, the oil conduit 446, theports 126d and 126e of the 2-4 timing valve 26, and the orifice 602.Therefore, the oil pressure in the clutch C3 is restricted by theorifice 602 and thus discharged gradually during the initial stage. Whenthe oil pressure in the clutch C2 rises and exceeds a predeterminedvalue, the 2-4 timing valve 26 is urged to move upward to the right halfposition thereof. When the 2-4 timing valve 26 assumes this position,the oil pressure in the clutch C3 which has been discharged via the port126e starts to be discharged via the port 126c and thus is notinfluenced by the flow restriction by the orifice 602. Thus, the oilpressure in the clutch C3 drops rapidly. Since this permits the clutchC3 to be disengaged after the clutch C2 initiates its engagement, agreat shift shock or engine racing is prevented from taking place. Sincethe predetermined value of the oil pressure in the clutch C2 at whichthe 2-4 timing valve 26 is moved from the illustrated left half positionthereof to the right half position thereof rises as the throttlepressure rises, the clutch C3 is disengaged early as the depressiondegree of the accelerator pedal decreases, thus providing a short periodof time during which the neutral state is maintained. During this periodof time, the engine revolution speed drops to a level corresponding tothe current vehicle speed, thus alleviating the shift shock.

Although, in the above description, the upshift operation between twoadjacent speed ratios is described, the downshift operation between twoadjacent speed ratios will also be readily apparent.

When the governor pressure drops or the throttle pressure rises duringtravelling with the fourth speed ratio, the 3-4 shift valve 24 is movedfrom the upshift position thereof to the downshift position thereof,admitting the line pressure in the oil conduit 412 to the clutch C3 viathe oil conduit 442 so as to engage same, admitting the line pressure inthe oil conduit 434 to the servo release chamber S/R so as to releasethe brake B2. As a result, the third speed ratio is established by theengagement of clutch C2 and that of the clutch C3. Owing to the functionof the 4-3 accumulator 40, the oil pressure in the clutch C3 is allowedto increase gradually. That is, although in the fourth speed ratio statethe piston 240 of the 4-3 accumulator 40 is pressed down by the linepressure in the chamber 140a because the oil pressure in the chamber140b is discharged via the oil conduit 442, the piston 240 is urged tomove upward by the spring 340 as the oil pressure builds up in the oilconduit 442 after the 3-4 shift valve 24 has been moved to the downshiftposition thereof. This upward movement of the piston 240 permits the oilpressure in the oil conduit 442 (i.e., the oil pressure in the clutchC3) to rise gradually. Since the clutch C3 is engaged during thisgradual rise in the oil pressure, the clutch C3 is engaged with theappropriate bias force, thus preventing a great shift shock from takingplace.

When the governor pressure drops further or the throttle pressure risesfurther during travelling with the third speed ratio, the 2-3 shiftvalve 22 is urged to move from the upshift position thereof to thedownshift position thereof, allowing the oil pressure in the oil conduit434 to be discharged to the drain port 122h. Then, the oil pressurewhich has acted on the clutch C2 disappears and the clutch C2 isdisengaged, while the brake B2 is engaged because the oil pressure inthe servo release chamber S/R of the brake B2 is discharged via the oilconduit 444, ports 124d and 124e and the oil conduit 434. Therefore, thesecond speed ratio is established by the engagement of the clutch C3 andthat of the brake B2. The discharge of the pressurized oil from theservo release chamber S/R is controlled by the 3-2 timing valve 32. Thatis, since the 3-2 timing valve 32, the check valve 754 and the orifice654 are arranged in parallel in the midway in the oil conduit 434extending from the port 124e of the 3-4 shift valve 24 and the port 122gof the 2-3 shift valve 22, the ports 124e and 122g are communicated witheach other bypassing the orifice 654 when the 3-2 timing valve 32assumes the illustrated left half position thereof, while when the 3-2timing valve 32 assumes the illustrated right half position thereof,these ports 124e and 122g are communicated with each other via theorifice because the direct communication between them is shut off. The3-2 timing valve 32 assumes the illustrated left half position thereofwhen the force due to the throttle pressure acting on the port 132 e islarger than the force due to the governor pressure acting on the port132a (i.e., in acceleration state), while in the reverse case (i.e., incoasting state) the 3-2 timing valve 32 assumes the left half positionthereof. Since, in acceleration state, the oil pressure in the servorelease chamber S/R is discharged gradually through the orifice 654, theoil pressure in the servo release chamber S/R drops gradually. Thiscauses a slight delay in the engagement of the brake B2 (the oilpressure in the clutch C2 is discharged quickly via the check valve 750in the oil conduit 434), thus providing a neutral state although veryshort, allowing the engine to increase its speed to the levelcorresponding to the vehicle speed. Thus, the variation in enginerevolution speed which would take place during the shift is suppressed,thus alleviating the shift shock.

When the governor pressure drops further or the throttle pressure risesfurther during travelling with the speed ratio, the 1-2 shift valve 20is moved from the upshift position thereof to the downshift positionthereof, allowing the oil pressure in the oil conduit 432 to bedischarged toward the drain port 120i. As a result, the oil pressureacting on the servo apply chamber S/A disappears, releasing the brakeB2. Thus, the first speed ratio is established by the engagement of theclutch C3 and the action of the one-way clutch OWC.

The operation under kickdown condition when the accelerator pedal isdepressed deeply beyond seven eighths (7/8) of the full stroke thereofis described.

When the accelerator pedal is depressed deeply enough, the plunger 207of the throttle valve 6 is pushed inwardly in the rightward direction tothe illustrated upper half position thereof, rendering the throttlevalve 6 inoperative, allowing the line pressure to be supplied to theoil conduit 420. The line pressure supplied from the oil conduit 420 tothe port 108a is supplied past the port 108e to the oil conduit 418which serves as the kickdown pressure circuit. The line pressure in theoil conduit 418 is supplied to the port 120b of the 1-2 shift valve 20and to the port 122c of the 2-3 shift valve 22, and also to the port124b of the 3-4 shift valve 24 through the shuttle valve 508 and the oilconduit 440. When the line pressure is supplied to the port 124b of the3-4 shift valve 24, the plug 225 is pressed downward. Since the governorpressure can not be greater than the line pressure, the spool 224 isalways held in the downshift position thereof under the kickdowncondition. Thus, whenever the accelerator pedal is depressed to producethe kickdown condition during travelling with the fourth speed ratio,the transmission always shifts down to the third speed ratio anddownwards and is prevented from shifting up from the third speed ratioto the fourth speed ratio. The kickdown pressure (line pressure)supplied to the port 122c of the 2-3 shift valve 22 acts on thedifferential area between the lands 222a and 222b of the spool 222,pressing the spool 222 downward. Since the additional downward force isapplied to the spool 222, the 2-3 shift valve 22 can not be shifteduntil the governor pressure creating the opposed upward force rises highenough to offset the additional downward force. That is, in the kickdownstate, a vehicle speed value at which the an upshift from the secondspeed ratio to the third speed ratio and that at which a downshift fromthe third speed ratio to the second speed ratio take place increasegreatly as compared to the vehicle speed values in the non-kickdownstate. The kickdown pressure in the oil conduit 418 is also supplied tothe port 110b of the throttle modulator valve 10, rendering the throttlemodulator valve 10 inoperative, allowing the line pressure to develop inthe oil conduit 424 which was supplied with the throttle modulatorpressure. Therefore, since the line pressure is supplied to the port122d of the 2-3 shift valve 22 and the line pressure is also supplied tothe port 122e thereof (this line pressure having replaced the throttlepressure as a result of kickdown state), the same oil pressure acts onthe differential area between the lands 222b and 222c of the spool 222when the spool 222 assumes the upshift position thereof and when thespool 222 assumes the downshift position thereof, so that a differencebetween the governor pressure value at which the 2-3 shift valve 22upshifts and that at which the 2-3 shift valve 22 downshifts decreases.In other words, the hysteris between the 3-2 downshift and 2-3 upshiftbecome small in the kickdown state. The kickdown pressure supplied tothe port 120b of the 1-2 shift valve 20 acts on the differential areabetween the lands 220a and 220b when the spool 220 is in the downshiftposition thereof, and acts on the differential area between the lands220a and 220b and the differential area between the lands 220b and 220cwhen the spool 220 is in the upshift position thereof, urging the spool220 downward. Therefore, the 1-2 shift valve 20 does not shift until thegovernor pressure increases by an additional value which offsets theadditional downward force due to the kickdown pressure. In other words,in the kickdown state, a vehicle speed value at which the 1-2 upshifttakes place and that at which the 2-1 downshift takes place increasegreatly as compared to such vehicle speed values in the non-kickdownstate.

The relationship between the vehicle speed and the throttle openingdegree at which the automatic shifts take place thus far described isillustrated by the shift diagram as shown in FIG. 9.

The operation of the overdrive inhibitor solenoid 42 is described. Asdescribed before, when the solenoid 42 is turned on, the line pressurebuilds up in the oil conduit 409. The oil pressure in the oil conduit409 passes through the shuttle valve 504, shuttle valve 508 and oilconduit 440 to reach the port 124b of the 3-4 shift valve 24, urging theplug 225 upward and the spool 224 downward. Thus, irrespective of themagnitude of the governor pressure, the 3-4 shift valve 24 is held inthe downshift position thereof, preventing upshift to the fourth speedratio. Therefore, if the driver does not wish the fourth speed ratio(overdrive) for the travelling condition, what he or she has to do is tomanipulate the overdrive inhibitor switch SW to prevent the automatictransmission from operating with the fourth speed ratio.

The operation in the case where the manual valve 4 has been moved to the"II" position during travelling with the third or fourth speed ratiowith the manual valve 4 set in "D" position is described.

When the manual valve 4 is placed in the "II" position, since the linepressure develops in the port 104d in addition to the port 104e, theline pressure is supplied to the oil conduit 414. The line pressure inthe oil conduit 414 passes through the shuttle valve 504, shuttle valve508 and oil conduit 440 to the port 124b of the 3-4 shift valve 24. Whenthe line pressure acts on the port 124b, the spool 224 assumes thedownshift position thereof for the same reason which was described inconnection with the kickdown condition and the operation of theoverdrive inhibitor valve 42. The line pressure in the oil conduit 414is also admitted to the port 122a of the 2-3 shift valve 22, acting onthe upper end of the plug 223 to urge it to move downward to theillustrated right half position thereof. As a result, the automatictransmission is forced to shift to the second speed ratio irrespectiveof the vehicle speed and thus is prevented from shifting to the thirdspeed or fourth speed ratio.

The line pressure in the oil conduit 414 is admitted to the port 116e ofthe line pressure booster valve 16. Since the line pressure boostervalve 16 assumes the lower half position thereof owing to the oilpressure acting on the port 116b, the oil pressure in the port 116e isadmitted to the port 116d and thus the line pressure develops in the oilconduit 428. This causes the line pressure to increase to its peak valueirrespective of the opening degree of the throttle valve, so that brakeB2 which is a band brake is applied strongly enough. Thus, a shift tothe second speed ratio is effected swiftly and effective engine brakingis provided without any delay.

Since the relationship between pressures acting on the 1-2 shift valvein this case is quite the same as in the case when the manual valve 4 isin the "D" position thereof, the 1-2 shift valve 20 is movable dependingupon the relation of the magnitude of the governor pressure to that ofthe throttle pressure. Therefore, the automatic shift between the firstand second speed ratios is carried out even when the manual valve 4 isplaced in the "II" position.

Although, as mentioned above, the line pressure is increased to its peakvalue irrespective of the throttle opening when the manual valve 4 isplaced to the "II" position from the fourth speed ratio or the thirdspeed ratio, the line pressure decreases in the following process to thesame level as would be when the manual valve 4 is in the "D" positiononce the automatic transmission shifts to the first speed ratio. Theshift from the second speed ratio to the first speed ratio (i.e., whenthe 1-2 shift valve 20 moves to the downshift position thereof) causesthe oil pressure in the oil conduit 432 to be discharged to the port120i. Thus, the oil pressure acting on the left end of the spool 216 ofthe line pressure booster valve 16 after passing through the port 116band the bore 216d of the spool 216 disappears, allowing the spool 216 tomove to the upper half position thereof owing to the action of thespring 316, shutting off the communication between the oil conduit 414and the oil conduit 428, allowing the oil pressure in the oil conduit428 to be discharged to the port 116c. Thus, the pressure modifier valve12 and the cut back valve 14 operate in the same manner as they do whenthe manual valve 4 is in the "D" position. In this circumstance, even ifthe 1-2 shift valve 20 is moved to the upshift position thereof againand the oil pressure develops in the oil conduit 432, the port 116b ofthe line pressure booster valve 16 is blocked by the land 216b of thespool 216, so that the line pressure booster valve 16 is held in theillustrated upper half position thereof. Therefore, even if the secondspeed ratio is selected again, the line pressure will not increase toits peak value. With this function of the line pressure booster valve16, the shift shock upon a shift between the first speed ratio and thesecond speed ratio in the "II" position is suppressed as low as that inthe "D" position.

When the manual valve 4 is moved to the "I" position, the line pressuredevelops in the port 104f as well as in the ports 104d and 104e,allowing the line pressure to develop in the oil conduit 416. The linepressure in the oil conduit 416 is admitted to the port 134d of themanual first range pressure reducing valve 34. This line pressure passesthrough the port 134c and the oil conduit 448 to reach the port 134e,urging the spool 234 upward to an equilibrium state wherein the spool234 slightly opens the drain port 134b. Thus, the oil pressure in theport 134e (i.e., the oil pressure in the oil conduit 448) will have aconstant value which is lower than that of the line pressure anddetermined by the force of the spring 334. This constant pressure in theoil conduit 448 is admitted via the shuttle valve 502 and the oilconduit 426 to the port 120d of the 1-2 shift valve 20, acting on theupper side of the land 221a of the spool 221, urging the spool 221 tomove downward and the spool 220 to move upward. As a result, as long asthe governor pressure urging the spool 221 upward is less than apredetermined value, the spool 221 is held in the downshift positionthereof (while the spool 220 is in the up position thereof). In thisposition of the spool 221, the line pressure in the oil conduit 432 isdischarged to the drain port 120i, thus releasing the brake B2. At thesame time, the port 120d of the 1-2 shift valve 20 is allowed tocommunicate with the port 120e thereof, allowing said constant pressurein the oil conduit 436 to be supplied to the brake B1. Therefore, thefirst speed ratio is established by the engagement of the clutch C3 andthe engagement of the brake B1 where the effective engine braking isprovided. Since if the governor pressure is greater than saidpredetermined value, the spool 221 of the 1-2 shift valve 20 does notmove to the downshift position thereof even when the constant pressurefrom the first manual range pressure reducing valve 34 acts on the spool221, the transmission does not shift down to the first speed ratio aslong as the vehicle speed is higher than a predetermined value even ifthe manual valve 4 is placed to the "I" position, thus preventing theengine overrun.

The operation in the case when the manual valve 4 is moved from the "N"position to the "R" position is described. When the manual valve 4 isplaced to the "R" positon, the line pressure develops only in the port104b, and this line pressure is admitted via the oil conduit 408 to theclutch C1, and on the other hand it is admitted via the oil conduit 408,shuttle valve 502, and oil conduit 436 to the port 120d of the 1-2 shiftvalve 20. With this oil pressure acting on the port 120d, the spool 221of the 1-2 shift valve is always shifted to the downshift positionthereof (there is no governor pressure developed because no linepressure exists in the oil conduit 412), so that the oil conduit 436 isallowed to communicate with the oil conduit 438 so as to supply the linepressure to the brake B1. Therefore, the reverse drive is established bythe engagement of the clutch C1 and that of the brake B1.

As described before, since, according to the present invention, the 4-3accumulator 40 is fluidly disposed in the oil conduit 442 communicatingwith the clutch C3 at a portion downstream of the orifice 658 (and alsodownstream of the check valve 758), the oil pressure supplied to theclutch C3 rises gradually during the movement of the piston 240 when theoil pressure is supplied to the clutch C3 through the oil conduit 442.Because the oil pressure is supplied to the clutch C3 when the manualvalve 4 is moved from the "N" position thereof to the "D" positionthereof or during a downshift from the four speed ratio to any one ofthe third speed ratio, second speed ratio and first speed ratio, theshock taking place during each of the shift is alleviated.

I claim:
 1. In an automatic transmission:a plurality of fluid operatedfriction units which are selectively made operative and inoperative toestablish, in a forward drive range, a plurality of forward speed ratiosincluding a highest speed ratio and other forward speed ratios, saidplurality of fluid operated friction units including a fluid operatedclutch which is to be engaged when the transmission downshifts from thehighest speed ratio to any one of the other forward speed ratios andwhen the transmission shifts from a neutral position to the forwarddrive range; a source of fluid pressure; means, including a fluidconduit communicating with said fluid operated clutch, for supplying thefluid pressure from said source of fluid pressure to said fluid operatedclutch through said fluid conduit when any one of the other forwardspeed ratios is established; a pressure accumulator including a bore, afluid operated piston movably disposed in said bore to form within saidbore a first chamber on one side of said piston and a second chamber onan opposite side of said piston, and a spring disposed in said secondchamber biasing said piston in a first direction, said first chambercommunicating with said source of fluid pressure, said second chambercommunicating with said fluid conduit, said piston being urged to moveby said spring when said second chamber is supplied with the fluidpressure from said fluid conduit with said first chamber being suppliedwith the fluid pressure from said source of fluid pressure when saidfluid operated clutch is to be engaged when the transmission downshiftsfrom the highest speed ratio to any one of the other forward speedratios or when the transmission shifts from the neutral position to theforward drive range in order to retard a rise in fluid pressure suppliedto said fluid operated clutch through said fluid conduit.